To multiply together 2-digit numbers having 5 as the tens digit: ex. 5 3 x 51 = 2703
-Multiply together the ones digits of the numbers being multiplied. This yields the last 2 digits of the answer. ex. 3 x 1 =_ _ 03
-Add the ones digits of the numbers being multiplied and divide by two. If this “average” digit is a whole number add it to the square of the common tens digit (5) of the numbers being multiplied. This yields the first 2 digits of the answer. ex. (3 + 1)/2 = 2 5 x 5 + 2 = 27_ _
*If the “average” digit above is not even, it is not even by half a hundred. Use this “average” figure disregarding the fraction, but adding 50 to the answer. ex. 52 x 57 = 2964, where (2 + 7)/2 = 4 and fraction. Adding this 4 to the square of the common tens digit (5) yields 29 for the first 2 numbers of the answer. Because the “average” figure is off by half a hundred, add 50 to the last 2 digits of the answer.
*When doing this type of problem, know that if both ones digits in the problem are even , or if both are odd, the “average’ will be a whole number and no problem exists. If the ones digits in the problem are a combination of even and odd, only then is 50 is added to the answer.
Consider: 54 x 54 = 2916 58 x 52 = 3016 51 x 58 = 2958 53 x 54 = 2862 52 x 59 = 3068
